En route, Krell's cruiser detected the newly refitted USS Enterprisewarping out of the Sol system. Realizing the potential advanced technologies incorporated into the starship, Krell ordered his ships to pursue the Enterprise to the Scylla sector, where they ambushed the Constitution-class vessel and ordered Captain Kirk to surrender his ship. Unfortunately, Krell was forced to abandon the assault when the Enterprise tested the "omegatron device" on his ship and crippled her. (TOS - Untold Voyagescomic: "Renewal")

The Krell who appeared in TOS - Untold Voyagescomic: "Renewal" wasn't associated with the Krell who appeared in TOSepisode: "A Private Little War", but with no contradictory evidence, it has been assumed that they are the same person. The fact that Krell had cranial ridges in "Renewal" could be resolved by the fact that, like many QuchHa' Klingons, the cure that was developed in the 2270s was successful on him or he had ridges surgically added, as revealed in STnovel: Excelsior: Forged in Fire.